It's not impossible to hack the hardware to do this: the output of the SDRAMs go to some 74-series chips which drive both the DAC and the connector for the 16-bit output.
You could remove the SDRAM path altogether, reverse the signal path from the connector with some PCB surgery and drive the DAC from the connector with your own digital hardware. That's one way...
There's also a hardware path from the ISA bus straight to the DAC, bypassing the SDRAM, for diagnostic purposes probably. So there might by options there too, but though it would require a ton of reverse engineering or even reprogramming the FPGAs (which are so old that the software isn't available anymore on the Altera website.)
Whether or not all of that is worth doing for a 14-bit DAC output is a matter of personal opinion.
A 14-bit dac at 105msamples/s with delta sigma modulation means you can get an effective bit depth of ~50 bits @20khz which is a ridiculous amount of dynamic range. Far more than what is implied by "just" 14-bits.
You could remove the SDRAM path altogether, reverse the signal path from the connector with some PCB surgery and drive the DAC from the connector with your own digital hardware. That's one way...
There's also a hardware path from the ISA bus straight to the DAC, bypassing the SDRAM, for diagnostic purposes probably. So there might by options there too, but though it would require a ton of reverse engineering or even reprogramming the FPGAs (which are so old that the software isn't available anymore on the Altera website.)
Whether or not all of that is worth doing for a 14-bit DAC output is a matter of personal opinion.